Processes for the alkylation of aromatic hydrocarbons are well known. Such processes typically utilize an aromatic reactant such as benzene or toluene which is reacted over a solid catalyst with an aliphatic alkylating agent that contains a straight chain paraffin moiety such as a methyl or ethyl group which can be in such forms as a paraffin, an alcohol, or a chloride. The process of alkylating aromatic hydrocarbons, or alkylation in general, involves the evolution of large amounts of heat. As a result, it is commonplace for an alkylation process to incorporate means for cooling the reaction zone or reactants in an alkylation process. Obtaining maximum performance in a multiple reaction zone process for the alkylation of aromatic hydrocarbons places a number of demands on the composition and properties of suitable quench medium. First, certain quench mediums such as liquid water or steam are unacceptable for many aromatic alkylation processes since it is known that liquid water, and in many cases steam, can permanently deactivate the catalyst by dealumination of catalyst supports or other destructive reactions. Water or steam can also pose other problems with zeolitic catalyst supports unless special precautions are taken to protect the zeolite matrix from unwanted cations such as sodium. In addition, when a series of reaction zones are used to alkylate aromatic hydrocarbons, deactivation of the catalyst will usually occur at different rates in the different reaction zones. Therefore, wide variations in the quench requirements exist for the different reaction zones. As a result, whatever quench medium is used, it must have sufficient heat capacity to effect the desired temperature control in each reaction zone. Therefore, regardless of the composition of the quench, the heat capacity of the quench is normally a fixed variable in the operation of an exothermic process and the volume of the quench is varied to obtain the desired degree of cooling. This is normally the case whether the heat capacity of the quench is based on the specific heat of the quench material alone or also includes a phase change for additional heat absorption. In order to be sure that adequate quench capacity is available, a quench stream is selected that can be added to a reaction zone in sufficient quantity. Whenever a quench medium is added to a reaction zone in a quantity greater than that required for the reaction occurring therein, it imposes additional cost to the process in the form of separation facilities and utilities.